This invention relates to a method of producing blends of thermoplastic polymers and high viscosity silicone fluids which contain temperature sensitive additives. More particularly, this invention relates to a one-step process for producing a thermoplastic/silicone fluid blend containing additives sensitive to the blending temperature.
Blending thermoplastic polymers with silicone fluids often provides blends with desirable engineering properties and improved flame retardance, such as where the silicone fluid is part of a flame retardant package. Examples of such desirable blends are disclosed by McLaury, et. al. in U.S. Pat. No. 4,273,691 and by Frye in U.S. Pat. No. 4,387,176; both disclosures being assigned to the same assignee.
Producing thermoplastic/silicone fluid blends with temperature sensitive additives has presented certain problems. The dispersion obtained when blending high viscosity silicone fluids and certain thermoplastic compositions has been found to be directly related to the magnitude of the temperature during blending. The limitation on the magnitude of the blending temperature is typically the temperature at which the thermoplastic polymers within the thermoplastic composition degrade. For such blends there is often an optimum blending temperature range where a high degree of dispersion is obtained with minimal polymer degradation. When blending at these optimum temperatures, blends with superior engineering properties are obtained. Often a desirable additive is sensitive to these optimum blending temperatures. An example of such an additive is the flame retardant aluminum trihydrate. To avoid loss of the additive's properties during blending, the blending temperature must be reduced. As a result, the engineering properties of the thermoplastic/silicone fluid blend suffer.
The present method of avoiding the loss of superior engineering properties and maintaining the integrity of the additive is to blend the high viscosity silicone fluid and thermoplastic composition at an optimum blending temperature in the absence of the temperature sensitive additive and then subsequently blend in the temperature sensitive additive at a lower temperature. This procedure is disadvantaged in that two separate blending procedures are required. To produce these thermoplastic/silicone fluid blends continuously, two extruders are required, i.e., twice the equipment required for conventional blending is utilized. Alternatively, the blend sample may be blended within one extruder twice, once at the optimum blending temperature and once with the sensitive additive. Therefore, either the rate of production suffers or the equipment required must be increased when sensitive additives are incorporated in the blend.
It is desirable to produce thermoplastic/silicone fluid blends containing additives sensitive to the optimum blending temperatures in a continuous, one-step procedure. The present invention is based on the discovery that the blend may be cooled within the extruder by feeding in a solid thermoplastic composition without significantly affecting the dispersion of blend constituents.